Greek philosopher Democritus believed that all matter was composed of very small indivisible particles. The term ‘atom’ comes from the Greek word “atomos” meaning indivisible.
Atoms are indestructible
The number of atoms is infinite.
Atoms could differ in size, shape and weight.
In 1803, English Chemist John Dalton was able to conduct an experiment that proved all elements are composed of small indivisible particles called atoms.
Specifically, Dalton’s theory specified:
Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
Atoms cannot be subdivided, created, or destroyed.
Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
In chemical reactions, atoms are combined, separated, or rearranged.
John Joseph (JJ) Thompson discovered that atoms were divisible, and and comprised of sub-atomic particles. He demonstrated the existence of negatively charged particles (electrons) and proposed the ‘plum pudding’ model of the atom.
The plum pudding model suggests that the atom is composed of positive material (the ‘cake’) with negative electrons scattered through it (the ‘fruit’). He believed the positive and negative charges balanced making atoms electronically neutral.
Scientist Ernest Rutherford disproved the plum pudding model with his gold foil experiment. This experiment determined that the atom is mostly empty space with a small dense positive centre (the nucleus) surrounded by diffuse negative charge (electrons).
Niels Bohr addressed the issues of the Rutherford model by suggesting that electrons orbit the nucleus at certain specific energy levels. These energy level levels have distinct, discrete radii and electrons can only change orbit/energy level by absorbing or releasing a quantum of energy suitable for that particular transition.
The quantum mechanical model of the atom synthesized the work of three prominent scientists - Louis de Broglie, Erwin Schrodinger & Werner Heisenberg. This model proposes that:
Electrons & other sub-atomic particles have a wave nature.
The allowed energies an electron can have and how likely it is for an electron to be found in various locations around the nucleus can be calculated from a mathematical equation.
An electron’s position cannot be precisely defined but only given a probability (found in electron ‘clouds’ of probability).
James Chadwick discovered that the nucleus was not solely made up of positive charges. He discovered the existence of the neutron, an electronically neutral sub-atomic particle located in the nucleus.